As a metal complex containing an anionic tridentate ligand, which does not belong to a cyclopentadienyl-based one, a complex having the following general formula (A) has been known (see Patent Document 1). It is reported that the metal complex has such a feature that the central metal M which is any transition metal of Groups 4 to 9 and being used as an olefin polymerization catalyst.

In addition, as a metal complex containing a tridentate ligand, but not a cyclopentadienyl-based one, a complex having the following general formula (B) has been known (see Non-Patent Document 1). Furthermore, a complex having the following general formula (B′) which includes tridante ligand same as tridante ligand of a complex as shown in the formula (B) has also been known (see Non-Patent Document 2).
However, there is no concrete report on reactivity of those metal complexes, and usages of those metal complexes are unknown.
(In the above formula, X represents chloro, methyl, ethyl, or trimethylsilylmethyl)
(In the above formula, X represents chloro, methyl, phenyl, or BH4, and Cp represents a cyclopentadienyl).
Furthermore, as a metal complex containing a tridentate ligand, but not a cyclopentadienyl-based one, a complex represented by the following general formula (C) has also been reported (see Non-Patent Documents 3, 4, and so on). It is known that the reported complex represented by the general formula (C) includes Pd, Ni, Pt, or the like as the central metal M thereof and part of the complex can be used as a catalyst component for the Heck reaction.
(In the above formula, M represents Pd, Ni, or Pt and X represents chloro or acetate).
On the other hand, an isoprene polymer, a butadiene polymer, an isoprene-styrene copolymer, a butadiene-styrene copolymer, a butadiene-isoprene copolymer, and so on are used as synthetic rubbers. In particular, a high-cis-1,4-isoprene polymer and a high-cis-1,4-butadiene polymer are synthetic rubbers that have almost the same strengths as those of natural rubbers and are very useful raw materials. Therefore, various investigations have been carried out for the process of producing an isoprene polymer or a butadiene polymer with a high cis-1,4 content. Furthermore, the isoprene polymer with a content of approximately 100% has been also known (see, for example, Patent Document 2 and Non-Patent Document 5).
Furthermore, it has been expected that when a polymer with a sharp molecular-weight distribution and a high-cis-1,4-isoprene polymer or a high-cis-1,4-butadiene polymer is produced, a higher-strength rubber can be provided.    Patent Document 1: JP 2002-513823 A    Patent Document 2: JP 2004-513998 A    Non-Patent Document 1: Michael D, Fryzuk et al. Organometallics 1996, 15, 3329-3336.    Non-Patent Document 2: Michael D, Fryzuk et al. Can. J. Chem. 2000, 15, 1003-1012.    Non-Patent Document 3: Mei-Hui Huang et al. Organometallics 2004, 23, 2813-2816.    Non-Patent Document 4: Lei Fan et al. Organometallics 2004, 23, 4778-4787.    Non-Patent Document 5: Shojiro Kaita et al. Macromolecules 2004, 37, 5860-5862.